Re: question about bd_inode hashing against device_add() // Re: [PATCH 03/11] block: call bdev_add later in device_add_disk

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2025/10/31 22:34, Greg Kroah-Hartman wrote:
> On Fri, Oct 31, 2025 at 08:23:32PM +0800, Gao Xiang wrote:
>>
>>
>> On 2025/10/31 18:12, Gao Xiang wrote:
>>> Hi Greg,
>>>
>>> On 2025/10/31 17:58, Greg Kroah-Hartman wrote:
>>>> On Fri, Oct 31, 2025 at 05:54:10PM +0800, Gao Xiang wrote:
>>>>>
>>>>>
>>>>> On 2025/10/31 17:45, Christoph Hellwig wrote:
>>
>> ...
>>
>>>>>> But why does the device node
>>>>>> get created earlier?  My assumption was that it would only be
>>>>>> created by the KOBJ_ADD uevent.  Adding the device model maintainers
>>>>>> as my little dig through the core drivers/base/ code doesn't find
>>>>>> anything to the contrary, but maybe I don't fully understand it.
>>>>>
>>>>> AFAIK, device_add() is used to trigger devtmpfs file
>>>>> creation, and it can be observed if frequently
>>>>> hotpluging device in the VM and mount.  Currently
>>>>> I don't have time slot to build an easy reproducer,
>>>>> but I think it's a real issue anyway.
>>>>
>>>> As I say above, that's not normal, and you have to be root to do this,
>> I just spent time to reproduce with dynamic loop devices and
>> actually it's easy if msleep() is located artificiallly,
>> the diff as below:
>>
>> diff --git a/block/bdev.c b/block/bdev.c
>> index 810707cca970..a4273b5ad456 100644
>> --- a/block/bdev.c
>> +++ b/block/bdev.c
>> @@ -821,7 +821,7 @@ struct block_device *blkdev_get_no_open(dev_t dev, bool autoload)
>>   	struct inode *inode;
>>
>>   	inode = ilookup(blockdev_superblock, dev);
>> -	if (!inode && autoload && IS_ENABLED(CONFIG_BLOCK_LEGACY_AUTOLOAD)) {
>> +	if (0) {
>>   		blk_request_module(dev);
>>   		inode = ilookup(blockdev_superblock, dev);
>>   		if (inode)
>> diff --git a/block/genhd.c b/block/genhd.c
>> index 9bbc38d12792..3c9116fdc1ce 100644
>> --- a/block/genhd.c
>> +++ b/block/genhd.c
>> @@ -428,6 +428,8 @@ static void add_disk_final(struct gendisk *disk)
>>   	set_bit(GD_ADDED, &disk->state);
>>   }
>>
>> +#include <linux/delay.h>
>> +
>>   static int __add_disk(struct device *parent, struct gendisk *disk,
>>   		      const struct attribute_group **groups,
>>   		      struct fwnode_handle *fwnode)
>> @@ -497,6 +499,9 @@ static int __add_disk(struct device *parent, struct gendisk *disk,
>>   	if (ret)
>>   		goto out_free_ext_minor;
>>
>> +	if (disk->major == LOOP_MAJOR)
>> +		msleep(2500);           // delay 2.5s for all loops
>> +
> 
> Yes, so you need to watch for the uevent to happen, THEN it is safe to
> access the block device.  Doing it before then isn't a good idea :)
> 
> But, if you think this is an issue, do you have a patch that passes your
> testing to fix it?

I just raise it up for some ideas, and this change is
buried into the code refactor and honestly I need to
look into the codebase and related patchsets first.

Currently I have dozens of other development stuffs
on hand, if it's really a regression, I do hope
Christoph or other folks who are familiar with the code
could try to address this.

Thanks,
Gao Xiang

> 
> thanks,
> 
> greg k-h